Find this building somewhere in the blooming plateaus or using the map. It will look like this:

Upon entering, you should see two bookshelves on the left and right. There are books here that contain some useful information about the Cobbleverse modpack itself.

After climbing the stairs, you will find Mars, who will need to defeat as a pre-requisite to Jupiter in the Team Galactic Eterna Building. Upon defeating Mars, you can pick up the Verity Proof, which can be put in a regional cartography table with an empty map to find Lake Verity, where Mesprit lives.